The present invention relates to a method for controlling the relative motion and force between two interconnected elements to minimize the instances of the motion exceeding acceptable limits, while maximizing isolation between the elements. More specifically it relates to a method for reducing excessive motion while achieving good isolation in a shock absorber or damper suspended system.
Vibration isolation mounts, such as primary vehicle suspensions, engine mounts, truck cab suspensions and truck and bus seat suspensions, isolate a "sprung mass" from the object on which the mass is mounted (the "unsprung" mass). For best isolation, the relative motion allowed by a mount should be large compared to the amplitude of the input vibrations to the unsprung mass. In most applications, however, the relative motion between sprung and unsprung mass must be limited to significantly less than the maximum possible input vibration amplitude. In primary vehicle suspensions the maximum allowable motion is determined by design constraints such as vehicle styling. In engine mounts and truck cab suspensions the maximum allowable motion is determined by the alignment of the engine or cab with other elements of the vehicle. In truck and bus seat suspensions the maximum allowable motion is determined by the ergonomics of the operator or passenger comfortably being able to reach elements of the unsprung mass while being isolated from that mass.
There are three general mount types: passive, semi-active and active. Passive mounts (engineered rubber mounts, springs with friction dampers, or, most commonly, springs with viscous dampers) are limited in performance due to compromises needed to achieve good control at the resonance frequency and good isolation at high frequencies. Active systems use sensors and control hardware and software to determine what forces are necessary to "cancel" the vibrations from the unsprung mass. These systems require a power source to provide the force needed for optimum control and isolation. Cost and performance limitations prevent active systems from being widely accepted. The semi-active systems also use sensors and control hardware and software to determine what actions are needed to achieve the desired control and isolation of the sprung mass. However, unlike the active systems, the semi-active suspensions do not use a power source for providing the control force. They use a controlled damper that can remove energy from the suspension system but cannot add energy to the system. Control algorithms are developed that allow a continuously-variable semi-active suspension to perform at a level of isolation that is comparable to the fully active system at a significantly reduced initial- and operating-cost and in a smaller and lighter package.
Several semi-active control algorithms have been proposed that use a controllable damper to achieve good isolation between sprung and unsprung masses. One such control algorithm uses the damper to counteract the force of the spring to limit the input force to the sprung mass. Another such control algorithm uses the damper to perform as if the damping is not between the sprung mass and the unsprung mass but is instead between the sprung mass and an inertial reference frame, the "sky". The "sky hook" model described by Karnopp, et al., "Vibration Control Using Semi-active Force Generator," ASME Paper No. 73-DET-123, May, 1974, is one of the best known models for a control algorithm. This reference discloses a damper which exerts a force tending to reduce the absolute velocity of the mass, while the conventional damper exerts a force tending to reduce relative velocity.
Guy, et al., "A Solenoid-Actuated Pilot Valve in a Semi-Active Damping System," SAE Paper No. 881139, August 1988, teaches that a shortcoming of the most effective isolation semi-active control algorithms, especially the "sky hook", is that large velocity and displacement inputs into the suspension can consume all available suspension travel, resulting in suspension "Topping" or "Bottoming."
This shortcoming must be overcome by using a) alternate algorithms that are less effective in isolating the sprung mass or b) control strategies that use the best vibration isolation algorithm when the probability of topping or bottoming is low, and use a separate algorithm, which acts to prevent the suspended mass from exceeding the stroke limits, when the probability of topping and bottoming is high.
U.S. Pat. No. 4,468,050, Woods et al., Aug. 28, 1984, discloses a computer optimized adaptive suspension system. This reference describes the problems of "Topping Out" and "Bottoming Out," i.e., the condition where a bump or other influence on the chassis or wheel causes the axle to try to rise toward the chassis closer than it can physically. This can cause a severe jolt to the passengers and possibly damage the shock absorber or suspension. To avoid this, the control process independently increases compression damping as the axle approaches bottoming out and increases rebound damping as the axle approaches topping out.
U.S. Pat. No. 5,276,622, Miller et al., Jan. 4, 1994, discloses a system for reducing suspension end-stop collisions, which provides an override control policy which alters the damper command signals as necessary to increase the damping characteristics of the damper assembly at times when the isolation system is likely to meet or exceed the end stops in order to minimize the incidence of end-stop collisions.